1. Field of the Invention
The present invention relates to a hydraulic drive system installed in, for example, construction machines.
2. Description of the Related Art
In a hydraulic drive system applied to an industrial machine, construction machine, or the like, hydraulic oil is supplied from a main pump to a plurality of hydraulic actuators. Specifically, a circulation line extends from the main pump to a tank, and a plurality of control valves are disposed on the circulation line. Each control valve controls hydraulic oil supply to and hydraulic oil discharge from a corresponding one of the actuators.
The control valves may be operated by pilot operation valves that output pilot pressures, or may be operated by operation devices that output electrical signals. In the latter case, generally speaking, each control valve receives pilot pressures outputted from a pair of solenoid proportional valves. For example, Japanese Laid-Open Patent Application Publication No. 2008-215420 discloses a hydraulic drive system 100 of a hydraulic excavator as shown in FIG. 9.
Specifically, in the hydraulic drive system 100, hydraulic oil is supplied from a main pump 110 to actuators 131 to 133 via control valves 121 to 123. Pairs of solenoid proportional valves (141, 142, and 143) output pilot pressures to the control valves 121, 122, and 123, respectively. Hydraulic oil from an auxiliary pump 150 is led to the solenoid proportional valves 141 and 142 through a first line 161. Hydraulic oil from the auxiliary pump 150 is also led to the solenoid proportional valves 143 through a second line 162.
The hydraulic drive system 100 adopts a configuration intended for handling failures of the solenoid proportional valves 141 to 143. (One of the valve failures to handle is, for example, any of the solenoid proportional valves getting stuck in the state of outputting a certain pilot pressure.)
Specifically, the first line 161 and the second line 162 are provided with a solenoid switching valve 171 and a solenoid switching valve 172, respectively. Among the pilot pressures outputted from the solenoid proportional valves 141 and 142, the highest pilot pressure is selected by high pressure selective valves 181 to 183, and the selected pilot pressure is measured by a pressure sensor 191. Similarly, a higher one of the pilot pressures outputted from the solenoid proportional valves 143 is selected by a high pressure selective valve 184, and the selected pilot pressure is measured by a pressure sensor 192. Then, if it is determined based on the measurement value of the pressure sensor 191 that a failure has occurred in the solenoid proportional valves 141 or 142, the solenoid switching valve 171 blocks the first line 161. If it is determined based on the measurement value of the pressure sensor 192 that a failure has occurred in the solenoid proportional valves 143, the solenoid switching valve 172 blocks the second line 162. In this manner, the supply of the hydraulic oil to the failed solenoid proportional valve(s) is stopped, and the control valve(s) corresponding to the failed solenoid proportional valve(s) is/are brought back to the neutral position.